This invention relates to a device for indicating whether or not a product has ever exceeded a predetermined temperature, and, more particularly, to such a device which is quite inexpensive in use and reliable, but does not require special handling prior to its association with a product which is to be monitored.
It is common practice in the food and medical industries, for example, to store perishables by freezing them until such time as they are to be used. For example, a significant part of any modern grocery store is devoted to frozen vegetables, juices, dinners, etc. It is also common to store blood plasmas in a frozen condition, and many vaccines are stored at reduced temperature.
It will be recognized that the unthawing or elevation of the temperature of a product prior to the time it is to be used, can affect the quality of such product. This is especially true of frozen unsterilized products, which normally contain bacteria and certain enzymes which cause deleterious changes to the product once it becomes unthawed. Unwanted thawing can occur during handling of a product. Such thawing and consequent product deterioration is often not detectable by the ultimate user of the product because of later refreezing. It will be recognized that use of a product, such as a blood plasma, certain foods and vaccines, after such undetected thawing or temperature elevation can have grevious results.
Because of the thawing-refreezing and temperature elevation problems, much effort has been devoted to providing a simple, effective and reliable means for indicating the thermal history of a product to a potential user so that assurance can be had that the product has not at any time attained such an elevated temperature that its quality is adversely affected. The paper entitled "Defrost Indicators" appearing in Food Technology, Vol. 26, pp. 46-50 (1972) provides a comprehensive list of patents and references setting forth the state of the art on this problem, with particular emphasis on frozen food thaw indicators. As pointed out in said paper, many thaw indicators have been designed in the past, and some have even been commercially produced and marketed. However, the ideal indicator does not exist, and none of those designed to date have been commercially successful.
Presently available thermal history indicators generally either require modification of the existing package for the frozen product so that the indicator can have access to the interior of the product while at the same time enabling checking from the exterior of such package, or merely measure the surface temperature of the product. Modification of the package is generally too expensive and cumbersome to be acceptable to the industry, and measurement of the surface temperature of the product is often not reflective of the state of the product as a whole. Because of such, neither approach is satisfactory. Another major difficulty with most present indicators emanates from the fact that they rely on the thawing of an indicating substance of one sort or another for their operation. Special handling has, therefore, been necessary during fabrication and later storage of the majority of such devices to assure that the indicating substance remains frozen, or such devices must be fabricated immediately prior to being applied to the product. While some devices include a membrane for containing the indicating substance, which membrane breaks to release the substance when the substance is frozen, such devices have not been reliable. That is, freezing may not always cause the membrane to break and release the diffusion agent. These problems have contributed to the reluctance of industry to adopt such devices.